Synthesis by sol–gel method and electrochemical properties of LiNi1−yAlyO2 cathode materials for lithium secondary battery

Single-phase LiNi1−yAlyO2 (y=0, 0.05, 0.1, 0.2, 0.25 and 0.3) powders with α-NaFeO2 structure are synthesized by sol–gel method using citric acid as a chelating agent, and LiNO3, Ni(NO3)2·6H2O and Al(NO3)3·9H2O as starting materials. Calcination is carried out at 800 °C for 13 h in oxygen stream after preheating at 600 °c for 5 h in air. The TGA and DTA curves show that the addition of Al decreases the weight loss rate and increases the temperature for the combustion of the residual organic matter and the formation of the LiNiO2 phase from the decomposition of Li2CO3 and NiO phases. The LiNiO2 particles are roughly in the form of smoothly edged polyhedra. As y increases, the particles become smaller. The LiNiO2 electrode has the largest first discharge capacity 168 mA h/g, and the discharge capacity 155 mA h/g at the 20th cycle. The shifting of the voltage range for intercalation and deintercalation to the higher voltage side leads to the decrease in the first discharge capacity of LiNi1−yAlyO2 in the fixed voltage range 3.0–4.2 V as y increases. The larger Δx and the occurrence of phase transitions at more times for y=0 are considered to bring about larger capacity fading with cycling. The smaller Δx and the occurrence of phase transitions at fewer times lead probably to the smaller capacity fading with cycling. Among the LiNi1−yAlyO2 (y=0, 0.05, 0.1, 0.2, 0.25 and 0.3) samples, LiNi0.95Al0.05O2 has the best electrochemical properties with a relatively large first discharge capacity 146 mA h/g and an excellent cycling performance.